1. Field of the Invention
The present invention relates to induction motor control generally and more specifically to fault isolation in an induction motor control system.
2. Description of the Related Art
Electrically-driven automobiles are often powered by a three-phase induction motor. That motor in turn often receives its power from a three-phase inverter connected to a battery. The inverter converts direct-current electric power from the battery into three-phase alternating-current electric power for use by the motor.
Inverters typically have a section dedicated to each phase of the motor. Each section has two switches, typically semiconductor switches. One side of the first switch is connected to positive voltage from the battery. One side of the second switch is connected to battery ground. The second sides of each switch are connected together. The node formed by this connection of the second side of each switch is the output of the particular section of the inverter in question. This output is connected to one phase of the motor.
All told, there are six semiconductor switches in an inverter, two for each of the three phases. Alternating current is produced for the motor through appropriate actuation of the switches, as is well-known in the art of induction motor control.
A difficulty occurs when one of the six switches in an inverter fails in a short-circuited condition. "Short-circuited" refers to the switch being continually closed. When any one of the six switches is short-circuited, the inverter can no longer control the induction motor. As long as the short-circuited switch remains connected to the motor, therefore, there is no other choice than to turn off the motor. The vehicle is then inoperative.
To allow the vehicle to operate, though in a degraded mode, a three-phase induction motor can be operated on only two phases. However, in order to operate the motor on two phases, the short-circuited switch must first be isolated from the motor.
One way to isolate a short-circuited inverter switch is disclosed in Fu and Lipo, "A Strategy to Isolate the Switching Device Fault of a Current Regulated Drive Motor" in Conference Record of the 1993 IEEE Industry Applications Society 28th Annual Meeting (1993). The system described in that paper is illustrated as FIG. 1. A pair of capacitors 22 and 24 are connected in series across battery 20, which is connected to inverter 21. Further, each circuit from inverter 21 to motor 35 has a fuse 32, 33 or 34 installed. Also, each circuit from inverter 21 to motor 35 is connected to the center node of capacitors 22 and 24 via a triac 36, 37 or 38. When a short circuit is detected on any one of the six switches 26, 27, 28, 29, 30 or 31 of inverter 21, the triac 36, 37 or 38 corresponding to that short-circuited switch is closed. The stored energy in capacitors 22 and/or 24 then blows the fuse connected to the triac. Thus, the short-circuited inverter switch is isolated from motor 35.
Although that system may be effective in isolating a short-circuited inverter switch, the use of capacitors in the traction system of an electric vehicle can be disadvantageous. To handle the high voltages of an electric vehicle, the capacitors would need to be quite large, adding expense to the vehicle. Further, a system which uses fewer overall components could potentially by more reliable and less costly.
Therefore, a system which can isolate a short-circuited inverter switch without the use of large capacitors and with fewer overall components can provide an advantage over the prior art.